Electroacoustic transducer

ABSTRACT

An electroacoustic transducer is described which is especially useful in telephones. An essentially flat frequency response curve in the voice range is realized by setting up a resonance peak in the free section of the diaphragm. The diaphragm is supported at its periphery by elastic bodies so that the fundamental frequency of oscillation of the diaphragm will have a node line in the region where the diaphragm is supported. The support arrangement according to the invention sets up at least a second node line having a smaller diameter than the fundamental node line and in the free section of the diaphragm.

United StatesPatent 1191 1111 3,708,702

Brunnert et al. 1451 Jan. 2, 1973 1541 ELECTROACOUSTIC TRANSDUCER3,297,968 1/1967 [75] Inventors: Otto Brunnert, Munich; Hans SchlerlEbersber both of Ger- I 3,085,167 4/l963 [73] Assignee: SiemensAktlengesellschatt, Berlin and Munich, Germany Primary Examiner-J. V.Truhe Assistant Examiner-B. A. Reynolds [.221 F'led: 1970Attorney-Birch, Swindler, McKie & Beckett [21] Appl.No.: 94,409

[57] ABSTRACT [30] 7 Foreign Application Priority Data 'An'electroacoustic transduceris described which is 7 especially useful intelephones. An essentially flat frequency response curve in the voicerange is realized by setting up a resonance peak in the free section of[52] US. Cl ..3l0/8.2, l79/3ll1g/9A23;2/08/.l86 diaphragm. h diaphragmis Supported at its [51] [m Cl mi 7/00 periphery by elast1c bod1es sothat the fundamental [58] Fieid 8 2 8 3 frequency of oscillat on of thediaphragm Wlll have a 9 h node line in the region where the diaphragm issupported. The support arrangement according to the invention sets up atleast a second node line having a DeclS, 1969 Germany ..P 19 612 217.0

[56] References Cited smaller diameter than the fundamental node lineand UNITED STATES PATENTS in the free section of the diaphragm.3,137,836 6/1964 Glover ..340/l0 8 Claims, 3 Drawing Figures 1 L 5 15 [1lfi 11 1/1 1 11 i 3 a 1/ //lLLl x14;

K I 1 1 I l B 12 13 15 5 9 21 \11 1 111 1 l 111 111 25 22 1ELECTROACOUSTIC TRANSDUCER BACKGROUND OF THE INVENTION This inventionrelates to electroacoustic transducers, and particularly to suchtransducers which utilize a diaphragm which will vibrate according tothe acoustic vibrations impinging thereon and produce correspondingelectrical signals. The invention is particularly applicable to thosediaphragms constructed of a material which requires that the diaphragmbe stressed to the point of bending.

It is known in transducers of the above type that the frequency responsecurves thereof in the bandwidth or frequency range in which it isdesired that the transducer operate meet certain predeterminedrequirements. These latter requirements particularly emphasize 'aconstant" or substantially'flat frequency response curve in the desiredarea of operation of the transducer. Prior art transducers are usuallyequipped with appropriately adjusted resonators that influence thefrequency response curve in the desired way by flatteningthe resonancepeaks thereon. Such transducers, however, are complicated and expensiveto manufacture.

Various forms of construction of transducers of this type have been usedin an effort to improve their characteristics. The prior artdemonstrates that diaphragms of electroacoustic transducers may besupported in their peripheral areas by the use of elastic intermediatelayers. For example, in carbon microphones the diaphragm is often fixedbetween two thin sheets of rubber. These sheets serve to insulate thediaphragm from the case of the transducer. This mode of construction,however, has a somewhat derogatory effect on the operation of thediaphragm, and in no way can be used to enhance its operationalcharacteristics, particularly its frequency response.

In electromagnetic transducers it is known to suspend the metaldiaphragm at its peripheral area between two rubber members havinglittle wall strength. The purpose of this suspension arrangement is toinsure that the diaphragm in each of its many possible positions is asflat as when it is at rest. This mounting arrangement, of course, has noapplication to diaphragms which are generally stressed to the point ofbending.

In constructing multiple layer diaphragms having several layers ofelectrostrictive material it is known to place the several layereddiaphragm between a firm and a resilient support around the peripheralportion thereof. This mode of suspension is useful for increasing thesensitivity of the transducer but it has no useful effect in producingthe desired frequency characteristics for the transducer.

It is, therefore, an object of this invention to provide a transducer ofthe type mentioned hereinabove which can have its frequency responsecurve easily adjusted to the desired flat characteristics in thefrequency band of interest.

It is another object of this invention to provide a support means forsuch a transducer which will cause the diaphragm of the transducer tohave a substantially flat frequency response curve in the frequencyrange in which it is desired that the transducer operate.

SUMMARY OF THE INVENTION The aforementioned and other objects arerealized in an electroacoustic transducer constructed according to theprinciples of this invention in which upon the diaphragm an additionalresonance peak is set up in addition to the resonance peak of thefundamental frequency of oscillation of the diaphragm. This additionalresonance peak is at a frequency which is numerically related to thefundamental frequency of the diaphragm and is within the desiredoperating range of the transducer, and the node line for the additionalpeak is in the free section of the diaphragm. This mode of operation ofthe diaphragm will produce a substantially constant frequency responsecurve in the frequency range in which the transducer is operating. Inthe preferred form of the invention the speech frequency range 'is ofinterest, and in prior art diaphragms operating in this frequency rangea substantial loss is usually experienced at the upper end of the range.The mode of operation described hereinabove avoids such losses in aninexpensive and uncomplicated manner.

The mode of oscillation for the diaphragm described hereinabove can besimply realized by placing the diaphragm between elastic positioningbodies having corresponding elastic characteristics. It has been foundthat, for example, silicon rubber is a material which is veryappropriate for this application. As discussed hereinabove, it is knownto use elastic materials in various forms to suspend a diaphragm, butthere is no suggestion in the prior art forms of the particular mode ofsuspension described herein which enhances the frequency characteristicsof a diaphragm of the type here in question. That is, the prior artmodes of suspension do not produce the additional oscillation node linein the free section of the diaphragm, and do not produce a flattening ofthe frequency response curve of the transducer in the range of interest.

The mode of elastic suspension of the diaphragm described hereinproduces a fundamental oscillation node line in the region of thesuspension, i.e., the diameter of the fundamental node linesubstantially corresponds to the diameter of the diaphragm. However, thenode line for the first harmonic of the fundamental frequency ofoscillation of the diaphragm will be in the free section of thediaphragm in an area adjoining the supported area. The parts of thediaphragm which are separated by the latter oscillation node line movein opposite directions. The material of the bodies supporting thediaphragm is deformed by the movement of the diaphragm in the area ofsuspension. This means that these resonance peaks are flattened in adesirable way and that any other resonance peaks are flattened therebysmoothing the frequency response curve.

The transducer according to this invention is particularly easy toconstruct in that the positioning bodies which touch the diaphragm onboth sides are extensions of a body of the same material that surroundsthe edges of the diaphragm or ofabutting bodies which meet outside ofthe edge of the diaphragm. This elastic body containing the diaphragm isthen supported within the transducer case. Elastic bodies constructedaccording to the invention and used with round diaphragms haveessentially the form of a ring split in its inside diameter and can beeasily placed against the diaphragm edge. The diaphragm and its elasticsupporting body can readily be positioned in a transducer case.

The elastic body supports the diaphragm by engaging with the portionsthereon radially inward of the edge of the diaphragm so that the edgearea of the diaphragm is free to move in a plane vertical of the planeof the diaphragm.

An advantageous mode of construction of the elastic supporting body isone that utilizes opposed projections extending inwardly of the elasticbody which engage with the opposite sides of the diaphragm. Inparticular, in the case of a round or disc-shaped diaphragm it has beenfound advantageous to use at least two ring shaped projections wherebythe radially outermost projection has a somewhat smaller diameter thanthe diameter of the diaphragm.

An elastic body having the desired elasticity can be realizedindependently from the size of such a body needed to fit within atransducer case by removing certain portions of the cross sectional areaof the body so that these portions of the elastic body are in factweaker.

It has been found that the projections extending from the elasticsupporting body to support the diaphragm can be made in a variety ofcross sectional shapes. Instead of using ring-shaped projections toengage the diaphragm, which are integral with the supporting body orbodies, it has been found that separated ring shaped supporting bodiesmade of elastic materials and having different diameters may be used.

The oscillatory behaviors of the diaphragm according to the invention isparticularly appropriate for electroacoustic transducers in which thediaphragm is provided with at least one layer of a material which haspiezoelectric or piezoresistive qualities. As is well known, suchdiaphragms are usually stressed to the point of bending according to theparticular characteristics of the material used. Depending on theapplication involved it is often useful to use a diaphragm made of metalhaving a layer of such a material. The fundamental frequency ofoscillation of such a diaphragm can be placed in the appropriate rangeaccording to the elastic qualities of the material used for thediaphragm.

BRIEF DESCRIPTION OF THE DRAWINGS The principles of the inventiondisclosed and claimed herein will be best understood by reference to adescription of the preferred embodiments given hereinbelow inconjunction with the drawings in which:

FIG. I is a cross sectional view of a piezoelectric microphoneconstructed according to the principles of this invention;

FIG. 2 is a partial cross sectional view of an alternative embodiment ofthe microphone illustrated in FIG. 1, and

FIG. 3 is a simplified cross sectional view of a suspended diaphragmaccording to the principles of the invention in the position for firstharmonic vibration.

DETAILED DESCRIPTION OF THE DRAWINGS The microphone in FIG. 1 includes aflat disc-shaped diaphragm 1 preferably made of titanium and which haspasted thereon a layer 2 of piezoelectric ceramic material. Thematerials used are, of course, a matter of choice depending on theapplication. The diaphragm I is contained in and supported by aring-shaped elastic body 3, preferably made of silicon rubber. Theelastic body 3 has two opposed pairs of inwardly projecting circular orring-shaped projections 6, 7, 8 and 9, which engage with and support thediaphragm 1. The radially outermost projections 6 and 8 are of adiameter such that the edge surface 14 of the diaphragm is free to movevertically in a plane perpendicular to the plane of the diaphragm.

The elastic body 3 containingthe diaphragm is supported within themicrophone structure upon a horizontally positioned disc 15 having anupward turning flange I6 around the outer edge thereof. The supportingmember 15 is itself supported by a case 18 of the microphone in the areaof the supporting member RE which is below the portion thereof whichsupports the elastic body 3 The assembly discussed above is covered by acover 117. The electrodes of the piezoelectric ceramic layer areconnected by leads 2H and 22 to an amplifier 23. This amplifier may beany of the wellknown types conventionally used for such applications andas is conventional as well, a diode 24 may be needed. The amplifier anddiode are suspended within a body of insulating material 25. This bodyof insulating material is fastened beneath the flange portions 16 of theabove mentioned disc-shaped supporting member 115.

The elastic supporting body 3 for the diaphragm is constructed so thatit extends upwardly to substantial engagement, or so that it at leastadjoins, the cover 17. This portion of the elastic supporting body isindicated by a numeral 4. The portion of the elastic body which rests onsupporting member 15 is indicated by the numeral 5. It has been foundthat by removing portions, such as portions 10 and 111, of the crosssectional area of the portion 4 of the elastic supporting body 3, and byremoving cross sectional portions 12 and 13 of the portion 5 of thesupporting body 3 that those portionsof the elastic supporting bodyhaving the smaller cross sectional area will substantially follow themotion of the diaphragm. The portions of body 3 which have this smallercross sectional area are vertically adjacent the projections 6-9. Thismode of construction allows the achievement of the desired elasticity ofthe supporting body substantially independently of the amount or size ofthe material which is needed to formthe elastic supporting body so thatit fills the area between the diaphragm and the remaining portions ofthe microphone assembly.

In the preferred embodiment of the invention described hereinabove,which is a microphone for use in telephones, the diaphragm has adiameter of 43 millimeters, and the piezoelectric material therein is ofdiameter of 30 millimeters with a thickness of 0.15 millimeters. Theopposed pairs of projections 6 and 8, and '7 and 9, of the elastic body3, respectively, have diameters of 41.5 and 38 millimeters.

In FIG. 2 is illustrated an alternative embodiment of the microphonediscussed in connection with FIG. 1. In this embodiment, an elasticsupporting body 26 for the diaphragm is formed in substantially the samemanner as is the body 3 in FIG. 1. However, the elastic body 26 does nothave the portions removed from its cross sectional area so that theportions 31 and 32 of the elastic body 26 do not have weaker areastherein. The elastic body 26 includes projections 27-30 which areconstructed in the manner described in FIG. 1, but in this case theprojections have a trapezoidal cross section. It has been found that anumber of cross sectional shapes are useful for this purpose.

FIG. 3 is a simplified cross sectional drawing illustrating theprinciples of operation of the invention. In this Figure, an elasticsupporting body L is shown in which a diaphragm M is supported in themanner described in connection with FIG. 1. The diaphragm M has apiezoelectric layer P fastened to the surface thereof. The Figureillustrates that in the free section of the diaphragm a nodal line isformed adjoining the suspended portions of the diaphragm. It is furthershown in this figure that the movement of the diaphragm deforms theelastic supporting body L in the illustrated manner during itsoperation.

The preferred embodiments of the invention described herein above areconsidered to be only exemplary, and it is contemplated that manymodifications or changes to these embodiments may be made within thescope of the dependent claims.

We claim:

1. An electroacoustic transducer, comprising:

a diaphragm constructed from electrostrictive material and means forsupporting said diaphragm around the periphery thereof constructed ofelastic material, said supporting means, in order to engage saiddiaphragm on both sides thereof, having at least an opposed pair ofprojections extending inwardly of said supporting means and around theperimeter thereof, the portions of said supporting means engaging saiddiaphragm on both sides thereof having corresponding elasticcharacteristics,

whereby a resonance peak corresponding to the fundamental frequency ofoscillation of said diaphragm, in the desired frequency range ofoperation of said transducer, and at least one additional resonance peakare set up, said additional resonance peak having a node line in thefree section of said diaphragm and being at a frequency which is in thefrequency bandwidth in which it is desired that said transducer operate.

2. The electroacoustic transducer defined in claim 1 wherein saiddiaphragm is stressed to the point of bendmg.

3. The electroacoustic transducer defined in claim 1 wherein saidelastic material is silicon rubber.

4. The electroacoustic transducer defined in claim 1 wherein saiddiaphragm is placed in said supporting means so that the edge area ofsaid diaphragm which vibrates in a plane perpendicular to the surfaceplane of the diaphragm is free of said supporting means.

5. The electroacoustic transducer defined in claim 1 wherein saidsupporting means comprises at least two opposed pairs of saidprojections having a shape substantially corresponding to the peripheralshape of said diaphragm and wherein the radially outermost of saidprojections surround a surface area of said diaphragm smaller than thetotal surface area of said diaphragm.

6. The electroacoustic transducer defined in claim 5 wherein saidsupporting means has portions removed from its cross sec ional area sothat the portions of said

1. An electroacoustic transducer, comprising: a diaphragm constructedfrom electrostrictive material and means for supporting said diaphragmaround the periphery thereof constructed of elastic material, saidsupporting means, in order to engage said diaphragm on both sidesthereof, having at least an opposed pair of projections extendinginwardly of said supporting means and around the perimeter thereof, theportions of said supporting means engaging said diaphragm on both sidesthereof having corresponding elastic characteristics, whereby aresonance peak corresponding to the fundamental frequency of oscillationof said diaphragm, in the desired frequency range of operation of saidtransducer, and at least one additional resonance peak are set up, saidadditional resonance peak having a node line in the free section of saiddiaphragm and being at a frequency which is in the frequency bandwidthin which it is desired that said transducer operate.
 2. Theelectroacoustic transducer defined in claim 1 wherein said diaphragm isstressed to the point of bending.
 3. The electroacoustic transducerdefined in claim 1 wherein said elastic material is silicon rubber. 4.The electroacoustic transducer defined in claim 1 wherein said diaphragmis placed in said supporting means so that the edge area of saiddiaphragm which vibrates in a plane perpendicular to the surface planeof the diaphragm is free of said supporting means.
 5. Theelectroacoustic transducer defined in claim 1 wherein said supportingmeans comprises at least two opposed pairs of said projections having ashape substantially corresponding to the peripheral shape of saiddiaphragm and wherein the radially outermost of said projectionssurround a surface area of said diaphragm smaller than the total surfacearea of said diaphragm.
 6. The electroacoustic transducer defined inclaim 5 wherein said supporting means has portions removed from itscross sectional area so that the portions of said supporting meanshaving a smaller cross sectional area are vertically adjacent saidprojections.
 7. The electroacoustic transducer defined in claim 1wherein said projections have a trapezoidal cross sectional shape. 8.The electroacoustic transducer defined in claim 1 wherein said diaphragmincludes at least a layer of piezoelectric material.